diff --git a/composer.json b/composer.json
index 6778b53cc..6fc33cc43 100644
--- a/composer.json
+++ b/composer.json
@@ -7,7 +7,7 @@
     "ext-openssl": "*",
     "ext-soap": "*",
     "google/auth": "^1.0.0",
-    "guzzlehttp/guzzle": "^7.0",
+    "guzzlehttp/guzzle": "^6.0 || ^7.0",
     "guzzlehttp/psr7": "^1.2",
     "monolog/monolog": "^2.2.0",
     "phpdocumentor/reflection-docblock": "^3.0.3 || ^4.0 || ^5.0",
diff --git a/src/Google/AdsApi/Common/AdsHeaderFormatter.php b/src/Google/AdsApi/Common/AdsHeaderFormatter.php
index facd9bf97..f8976b993 100644
--- a/src/Google/AdsApi/Common/AdsHeaderFormatter.php
+++ b/src/Google/AdsApi/Common/AdsHeaderFormatter.php
@@ -118,11 +118,20 @@ private function formatUtilUsages($adsUtilities, $includeUtilityUsage)
 
     private function formatGuzzleInfo()
     {
-        $guzzleInfoTokens = ['GuzzleHttp/' . ClientInterface::MAJOR_VERSION];
+        $guzzleInfoTokens = ['GuzzleHttp/' . $this->getGuzzleVersion()];
         if (extension_loaded('curl') && function_exists('curl_version')) {
             $guzzleInfoTokens[] = 'curl/' . \curl_version()['version'];
         }
 
         return implode(', ', $guzzleInfoTokens);
     }
+
+    private function getGuzzleVersion()
+    {
+        if (defined('GuzzleHttp\ClientInterface::MAJOR_VERSION')) {
+            return ClientInterface::MAJOR_VERSION;
+        }
+
+        return ClientInterface::VERSION;
+    }
 }
diff --git a/tests/Google/AdsApi/AdWords/Reporting/v201809/RequestOptionsFactoryTest.php b/tests/Google/AdsApi/AdWords/Reporting/v201809/RequestOptionsFactoryTest.php
index 6ddc19110..5dbb9a082 100644
--- a/tests/Google/AdsApi/AdWords/Reporting/v201809/RequestOptionsFactoryTest.php
+++ b/tests/Google/AdsApi/AdWords/Reporting/v201809/RequestOptionsFactoryTest.php
@@ -418,11 +418,20 @@ private function getFormattedUserAgent()
 
     private function formatGuzzleInfo()
     {
-        $guzzleInfoTokens = ['GuzzleHttp/' . ClientInterface::MAJOR_VERSION];
+        $guzzleInfoTokens = ['GuzzleHttp/' . $this->getGuzzleVersion()];
         if (extension_loaded('curl') && function_exists('curl_version')) {
             $guzzleInfoTokens[] = 'curl/' . \curl_version()['version'];
         }
 
         return implode(', ', $guzzleInfoTokens);
     }
+
+    private function getGuzzleVersion()
+    {
+        if (defined('GuzzleHttp\ClientInterface::MAJOR_VERSION')) {
+            return ClientInterface::MAJOR_VERSION;
+        }
+
+        return ClientInterface::VERSION;
+    }
 }